In a magnetic disk drive, a position servo system is apt to be shaken by the operation of the head actuator (during "track-seek" and/or "track following" operations), as well as by external forces such as wind. That is, in a magnetic disk drive position servo system, mechanical driving components can exhibit various resonances which de-stabilize, or impair, track following. As a result, servo operation can readily be upset by such vibration, especially at resonance points. Such resonances in the disk drive can cause the servo to become unstable. In general, oscillations can occur which severely impair or even destroy the control functions of the positioning servo loop. An object of this invention is to provide an improved servo loop including a resonance-attenuation filter which stabilizes and otherwise improves the positioning servo system in the presence of known mechanical resonances.
More particularly the invention prescribes a novel "switched capacitor" digital filter means provided in a "multiple-notch-filter" to attenuate resonances. This is preferably used as improved feedback control means and in particular improved filter means to attentuate prescribed band resonances.
In a preferred embodiment, a signal processing filter is taught which may be provided using off-the-shelf "switched capacitor filter" means. In the novel filter "second-order, low-pass" sections are summed with "second order, high-pass" sections to form a "fourth-order, double-notch filter". Such a filter is preferably arranged to have a depth (dB, or gain magnitude) adequate to attenuate one of a group of mechanical resonances which typically appear in a disk drive system. The filter may be set so that its complex poles and zeroes provide for minimum phase shift and maximum attenuation. This is as opposed to conventional filters featuring one or several single notch-filter means, each adapted to address only a single mechanical resonance frequency--being subject to the well known problems of frequency variance or filter variance whereby the filtering action can shift due to temperature and component tolerances.
Without such a filter, workers will appreciate that resonances in a disk drive system can render the servo instable, with oscillations causing the positioning loop to effectively lose control. Thus, our novel filter is characterized as improving the stability of a position servo system in the presence of a band of known mechanical resonances.
As workers know, for these and similar problems, the prior art has used analog filters which required capacitors whose operative values are subject to change with temperature and component tolerances; such are of questionable reliability and stability. Also more than one notch frequency was difficult to implement since component tolerances varied unpredictabley and could have an adverse effect on the phase shift of the filter.
The prior art has used single analog notch filters for each such resonance and these were subject to large tolerance variations; and they not only required multiple OP-amps but also external circuit capacitors to implement more than one notch, whereas with the invention multiple notches are accommodated in a single commercially-available device. Typically, such analog filter systems required one OP amplifier for each "notch"--whereas, with our invention, a plurality of notches can be handled within a single device. Thus, we characterized out invention as a "multiple-notch" filter, operating in a digital rather than analog manner.
As another preferred feature, the subject filter exhibits a local feedback loop adapted to correct any DC offset resulting from the switched capacitor means. As yet another feature, frequency tolerance is determined according to the stability of a master clock source, such as a crystal oscillator.
In sum, the subject filter can be used to attenuate several resonances across a multi-notch band and thus stabilize a closed-loop servo system. Such a filter can provide a prescribed gain reduction (e.g., 10.sup.+ DB or more) in the region of known head-arm resonance frequencies (e.g., about 3 to 5 KH.sub.z). Our filter is designed to provide minimum phase shift at the servo "open loop" gain crossover frequency. Such a filter is preferably implemented as a "fourth-order, elliptic-notch filter" means.
In any closed-loop servo system, one must take care that the filter phase shift does not combine with the phase shift of the overall drive, and thus compromise stability.
Therefore, my desired filter should operate so that its phase shift is a minimum at the open loop gain crossover frequency of the position servo system.